Laser interferometer displacement measuring system, exposure apparatus, and electron beam lithography apparatus

ABSTRACT

An absolute accuracy in the range from ±2 nm to ±1 nm for a displacement measurement value is provided by a laser interferometer displacement measuring system. A fluctuating error component that appears corresponding to the wave cycle of laser light is detected and subtracted from the measurement value while a stage is moving, thereby providing a high accuracy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to displacement measurement techniques,instrumentation techniques, evaluation techniques, precision patterningtechniques, fine patterning techniques, semiconductor patterningtechniques, and master mask patterning techniques. More particularly,the present invention relates to a displacement measurement techniquewhich requires accuracy of the order of nanometer.

2. Description of the Related Art

For example, a laser interferometer displacement measuring system isoften used as high accuracy displacement measurement means forcontrolling such as a stepper, employed in the photolithography processfor fabricating semiconductor devices, and for controlling X-Y stagesfor use such as in precision machining equipment. A nominal value ofresolution is 0.3 nm for the displacement measurement system whichprovides the most accurate displacement measurement and has beendeveloped particularly for stepping control.

Concerning the provision of increased accuracy, even a generaldisplacement measurement technique that does not employ the laserinterferometer displacement measuring technique but employs noiseprocessing by averaging is disclosed in Japanese Patent Laid-OpenPublication No. Hei 7-306034 in relation to the non-contact displacementmeasurement. Optical measurement employing an optical interferometer isdisclosed in Japanese Patent Laid-Open Publication No. Hei 9-178567 inrelation not to position but to wavelength measurement.

However, in many cases, even the current laser interferometerdisplacement measuring system having a nominal value of resolution of0.3 nm actually provides only an absolute accuracy of the order of ±2 nmfor displacement measurement. The resolution and the absolute accuracyare essentially different from each other. An interferometerdisplacement measuring system may apparently have an accuracy of 0.3 nmin the range of about 10 nm, but in some cases, a gradual undulation maybe generally found with the magnitude reaching 3 nm or more in the rangeof 100 to 300 nm. These problems were not made clear until a high-speedreal-time displacement measurement approach was developed to therebymake it possible to measure the displacement of a moving object withhigh accuracy at a frequency greater than that of mechanical vibrations.

In general, to provide an increased accuracy, the noise processing byaveraging over time is performed as mentioned above in order to improvethe accuracy (i.e., relative accuracy) of stability of measurementvalues under a standstill condition of the object. However, with arecent increasing demand for increased accuracy, the absolute accuracyof measurement values has become necessary. In the course of study tothe present invention, such a problem has become clear that theprior-art noise processing by averaging cannot provide a sufficientabsolute accuracy.

In view of the aforementioned problems, it is therefore the object ofthe present invention to provide a high-accuracy interferometerdisplacement measuring system which provides an absolute accuracy in therange of ±2 nm to ±1 nm or less for a displacement measurement valueusing interference of laser light.

SUMMARY OF THE INVENTION

In consideration of the fact that the interference of light itself,which is the principle of laser interferometry, causes an error, thepresent invention is adapted to eliminate errors, concerning absoluteaccuracy, which cannot be eliminated only by averaging over time.Approaches to increased accuracy that focus attention on such a cause oferror have never discussed before. More specifically, a correction valuecorresponding to the laser wave cycle of a displacement is added to adisplacement output of the laser interferometer displacement measuringsystem, thereby correcting the distortion error in the interferometerdisplacement measuring system.

Upon measurement of a continuously moving object as a measurementtarget, the laser interferometer displacement measuring system accordingto the present invention stores and corrects, as a measurement errorcaused by the interference effect, an oscillatory component that appearsin the cycle consistent with the frequency of laser light, therebyimplementing an increased accuracy. Even such a correction method forallowing a relatively simple sinusoidal wave to be added to orsubtracted from a measurement value can reduce the range of error ofabsolute position about ±2 nm to within ±1 nm, thereby making itpossible to provide an increased accuracy.

That is, a laser interferometer displacement measuring system accordingto the present invention is characterized by comprising a displacementmeasurement mechanism making use of laser interference, and correctormeans for adding a correction value to or subtracting the correctionvalue from a measurement value of the displacement measurementmechanism. The corrector means uses a cyclic correction value having acycle corresponding to a wave cycle of laser light.

Furthermore, a laser interferometer displacement measuring systemaccording to the present invention is characterized by comprising adisplacement measuring mechanism making use of laser interference, andcorrector means for adding a correction value to or subtracting thecorrection value from a measurement value of the displacementmeasurement mechanism. The corrector means has storage means for storinga cyclic correction value having a cycle corresponding to a wave cycleof laser light, and the correction value is read out of the storagemeans in accordance with the measurement value and is added to orsubtracted from the measurement value. It is possible to employ arewritable memory as the storage means.

Furthermore, a laser interferometer displacement measuring systemaccording to the present invention comprises a laser light source, aninterferometer for dividing laser light of wavelength λ emitted from thelaser light source into a reference path beam and a measurement pathbeam to interfere the reference path beam with the measurement path beamhaving been reflected from a subject body, a light detector fordetecting the light subjected to the interference in the interferometer,and measurement value output means for converting a detection signal ofthe light detector into a measurement value to output the resultingvalue. In the system, a displacement of the subject body causes ann-fold variation in length of an optical path between the interferometerand the subject body. The laser interferometer displacement measuringsystem is characterized by further comprising corrector means for addinga correction value to or subtracting the correction value from themeasurement value of the measurement value output means. The system isalso characterized in that, with the measurement value being employed asa variable, the corrector means uses, as the correction value, a cyclicfunction having a cycle of λ/n or a sum of a plurality of cyclicfunctions having the cycle of λ/n as a fundamental cycle. The pluralityof cyclic functions having the cycle of λ/n as a fundamental cycle canbe the cyclic function having a cycle of λ/n and harmonic cyclicfunctions thereof.

The aforementioned laser interferometer displacement measuring systemaccording to the present invention can comprise means for performingfeedback control so as to carry out tracking adjustment of a phase andamplitude of the correction value.

A laser interferometer displacement measuring system according to thepresent invention is characterized by comprising a displacementmeasurement mechanism making use of laser interference, and correctormeans for adding a correction value to or subtracting the correctionvalue from a measurement value of the displacement measurementmechanism. The corrector means prepares a plurality of types of cyclicfunctions having a cycle corresponding to a wave cycle of laser light,and assigns weights to each of the cyclic functions to allow theresulting cyclic functions to be added to or subtracted from themeasurement value.

It is also possible to employ mathematically orthogonal cyclic functionsas the plurality of types of correction value cyclic functions. Forexample, as the mathematically orthogonal cyclic function, it ispossible to use the sinusoidal (sin) and cosine (cos) functions and agroup of harmonics of these cyclic functions. In addition, as theplurality of types of correction value cyclic functions, it is possibleto use a triangular wave function and a group of orthogonal harmoniccyclic functions of the triangular wave. Incidentally, the plurality oftypes of correction value cyclic functions do not have necessarily to bemathematically orthogonal cyclic functions.

To use functions that are orthogonal to each other as the plurality oftypes of cyclic functions, the system can be provided with calculationmeans for calculating the magnitude of the component of each cyclicfunction in the cyclic error contained in a measurement value byintegrating each cyclic function individually with the measurement valueto perform the weighting by means of the output of the calculationmeans.

Furthermore, the system can be provided with phase shift means forshifting the phase of the plurality of types of cyclic functions at thesame time to perform feedback control on the amount of shift provided bythe phase shift means.

It is preferable to make the feedback time constant of the amount ofshift provided by the phase shift means shorter than the feedback timeconstant of other amplitudes. It is preferable to provide the systemwith means for enabling the feedback control only when the subject bodyis moving at a given speed or greater.

Furthermore, it is possible to dispose averaging means capable ofaveraging over time after the aforementioned corrector means. It is alsopossible to configure the system to bypass the averaging processingprovided by the averaging unit means for output.

Furthermore, a laser interferometer displacement measuring systemaccording to the present invention is characterized by comprising adisplacement measurement mechanism making use of laser interference, anderror signal component generating means for eliminating a constant speedcomponent and an acceleration component from a measurement value of thedisplacement measurement mechanism and generating an error signalcomponent. The system further comprises storage means for storing theerror signal component generated from the error signal componentgenerating means corresponding to the measurement value, and means forallowing the error signal component stored in the storage means to beadded to or subtracted from the measurement value of the displacementmeasurement mechanism as a correction value.

Furthermore, a laser interferometer displacement measuring systemaccording to the present invention comprises a laser light source, aninterferometer for dividing laser light of wavelength λ emitted from thelaser light source into a reference path beam and a measurement pathbeam to interfere the reference path beam with the measurement path beamhaving been reflected from a subject body, a light detector fordetecting the light subjected to the interference in the interferometer,and measurement value output means for converting a detection signal ofthe light detector into a measurement value to output the resultingvalue. In the system, a displacement of the subject body causes ann-fold variation in length of an optical path between the interferometerand the subject body. The laser interferometer displacement measuringsystem is characterized by further comprising corrector means for addinga correction value to or subtracting the correction value from themeasurement value of the measurement value output means. The correctormeans comprises means for storing or calculating, with the measurementvalue being employed as a variable, a cyclic function having a cycle ofλ/n or the cyclic function having a cycle of λ/n and harmonic cyclicfunctions thereof, and error signal component generating means foreliminating a constant speed component and an acceleration componentfrom a measurement value of the displacement measurement mechanism andgenerating an error signal component. The corrector means furthercomprises adjustment means for adjusting an amplitude and a phase of thecyclic function so that the cyclic function having a cycle of λ/n or asum function of the cyclic function having a cycle of λ/n and harmoniccyclic functions thereof fits to the error signal component, and meansfor allowing a function value of the cyclic function having a cycle ofλ/n or a function value of the sum function of the cyclic functionhaving a cycle of λ/n and harmonic cyclic functions thereof to be addedto or subtracted from the measurement value.

The system can be configured such that correction processing is carriedout by means of hardware in a real time manner or implemented bysoftware means in conjunction with a mechanism for calculatingcorrection values.

A laser interferometer displacement measuring system according to thepresent invention makes it possible to drive a displacement measurementsubject at a given speed upon activation or initialization of the systemto acquire correction data at that time. In addition, a laserinterferometer displacement measuring system according to the presentinvention may be adapted to be combined with a stage control device toset a correction value of a correction table for use in laserdisplacement measurement at the time of initial operation or control ofthe stage.

The measurement value correcting means or means for implementing themethod for correcting measurement values can be integrated on ameasuring board (counter board or axis board) for laser interferometry.

A laser interferometer displacement measuring system according to thepresent invention can be mounted on a single-axis stage, a multi-axisstage, or an X-Y stage.

An apparatus according to the present invention comprises a stage forplacing thereon and moving a sample or a subject work, drive means fordriving the stage, and a laser interferometer displacement measuringsystem for measuring a position of the stage. The apparatus ischaracterized in that, as the laser interferometer displacementmeasuring system, the aforementioned laser interferometer displacementmeasuring system is employed. Examples of those apparatuses include anelectron beam lithography system, a stepper for fabricatingsemiconductors (an exposure apparatus), a fine patterning system, metalmachining equipment, ceramic machining equipment, mask pattern transferequipment, mask patterning equipment, an electron-beam scanningmicroscope with a displacement measurement function, a transmissionelectron microscope with a displacement measurement function, andnon-contact shape measurement equipment.

A laser interferometer displacement measuring system according to thepresent invention comprises a light detector for detecting the lightsubjected to interference, phase detector means for detecting a phasefrom a detection signal of the light detector, accumulator means foraccumulating variations in phase obtained from the phase detector means,correction value generating means for generating a correction value froman accumulated value provided by the accumulator means or the phasevalue, and corrector means for allowing the correction value, generatedby the correction value generating means, to be added to the accumulatedvalue or the phase value. The correction value generating meansgenerates a cyclic correction value of wavelength λ of laser light withthe accumulated value or the phase value being employed as a variableand eliminates a signal component produced in phase with the wave cycleof the laser light.

The correction value generating means generates a correction value withan accumulated value, not a phase value, or variations in phase valuebeing employed as a variable, thereby generating a cyclic correctionvalue at a cycle of λ independent of n. The correction value isgenerated in the cycle of λ, thereby extracting the error of a pluralityof cyclic components corresponding to each harmonic component of 1 to 2n harmonic waves of wavelength λ.

The laser interferometer displacement measuring system according to thepresent invention comprises means for suppressing a relative peakintensity, with respect to a baseline of a frequency spectrum, of a peakof frequency component f=Nv/λ (N is a natural number of 1 to 2 n and notequal to n) of a signal generated in the light detector due to amovement of the subject body at speed v. This allows for eliminatingthose frequency components to provide increased accuracy for the laserinterferometer displacement measuring system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a view showing the principle of a correction method accordingto the present invention;

FIG. 2 is a view showing the overall configuration of a high accuracydisplacement measuring system according to the present invention,including a laser interferometer displacement measuring system and adriving system;

FIG. 3 is a view showing time-dependent variations in measurement valuewhen the displacement of a stage moving at a constant speed (5 mm persecond) is measured by a laser interferometer displacement measuringsystem without any correction;

FIG. 4 is a view showing time-dependent variations in measurement valuewhen the displacement of a stage moving at a constant speed (40 mm persecond) is measured by a laser interferometer displacement measuringsystem without any correction;

FIG. 5 is a view showing the flow of signals in an exemplaryconfiguration of distortion error corrector means according to thepresent invention;

FIG. 6 is a view showing an exemplary configuration of distortion errorcorrector means according to the present invention, in which the portionrequired for high speeds is constructed by hardware, showing the circuitconfiguration of the hardware portion and a feedback path;

FIG. 7 is a view showing an exemplary relatively simple configurationwhich allows for an automatic tracking correction in a distortion errorcorrection process according to the present invention;

FIG. 8 is a view showing the examples of two cyclic functions (shown bya solid and dotted line, respectively), orthogonal in phase to eachother, for use in automatic tracking distortion error correctionprocessing according to the present invention;

FIG. 9 is a view showing the examples of two cyclic functions (shown bya solid and dotted line, respectively), orthogonal in phase to eachother, for use in automatic tracking distortion error correctionprocessing according to the present invention;

FIG. 10 is a view showing an exemplary configuration of automatictracking distortion error correction processing according to the presentinvention, in which employed are two cyclic functions orthogonal inphase to each other;

FIG. 11 is a view showing an exemplary configuration of automatictracking distortion error correction processing according to the presentinvention, in which employed are three or more cyclic functionsorthogonal mathematically to each other;

FIG. 12 is a view showing an example of a group of cyclic functions foruse in the exemplary configuration of FIG. 11;

FIG. 13 is a view showing an exemplary configuration of automatictracking distortion error correction processing means according to thepresent invention, in which the amplitude control of a plurality ofcyclic functions orthogonal to each other is combined with the phasetracking control;

FIG. 14 is a view showing an example of a group of cyclic functions foruse in the exemplary configuration of FIG. 13;

FIG. 15 is a view showing the order of connection in the signalprocessing for the combination of distortion corrector means andaveraging means according to the present invention;

FIG. 16 is a view showing the frequency spectrum of a coordinatedisplacement signal with a stage being at a standstill before processingis performed by averaging means;

FIG. 17 is a view showing an exemplary configuration comprising meanswhich is referenced as an input to averaging processing only when acertain traveling speed of a stage is exceeded in the automatic trackingcorrection processing of distortion error;

FIG. 18 is a view showing the overall exemplary configuration in which alaser interferometer displacement measuring system according to thepresent invention is configured using phase measurement means(phasemeter);

FIG. 19 is a view showing the overall exemplary configuration of a laserinterferometer displacement measuring system for implementing acorrection method in which a correction value is generated employing aphase value as a variable to add the resulting correction value to thephase value;

FIG. 20 is a view showing the overall exemplary configuration of a laserinterferometer displacement measuring system for implementing acorrection method in which a correction value is generated employing aphase value as a variable to add the resulting correction value to anaccumulated value;

FIG. 21 is a view showing a laser interferometer displacement measuringsystem with a four-fold measurement path (n=4) in which an optical erroris occurring in the optical system;

FIG. 22 is a view showing a laser interferometer displacement measuringsystem with an eight-fold path (n=8) in which an optical error isoccurring in the optical system;

FIG. 23 is a view showing an actual example of a correction resultprovided when the sum of cyclic functions of 1 to 2 n harmonic waves isemployed as a correction value;

FIG. 24 is a view showing an exemplary configuration of an exposuresystem employing a laser interferometer displacement measuring systemaccording to the present invention;

FIG. 25 is a view showing an exemplary configuration of an electron beamlithography apparatus employing a laser interferometer displacementmeasuring system according to the present invention;

FIG. 26 is a view showing the entire exemplary configuration of a laserinterferometer displacement measuring system according to the presentinvention, which employs a method for performing automatic tracking ofand updating a correction value to provide increased accuracy;

FIG. 27 is a view showing the entire exemplary configuration of a laserinterferometer displacement measuring system according to the presentinvention, which employs a method for performing an automatic feedbackof a correction value to provide increased accuracy; and

FIG. 28 are views showing examples of frequency spectrum of ameasurement value; FIG. 28A showing an example without an accuracyimprovement method according to the present invention, FIG. 28B showingthe other example with the method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the present invention will be explained below with reference to theaccompanying drawings in accordance with the embodiments.

[Embodiment 1]

Basic Configuration of a Laser Interferometer Displacement MeasuringSystem with a Distortion Correction Function

The overall exemplary configuration of a laser interferometerdisplacement measuring system according to the present invention isshown in FIG. 2. In the figure, shown is an actual example in whichlaser displacement measurement is employed for measuring a single-axisstage with accuracy and performing feedback control. The single-axisstage has a motor or the like as a stage driving power source 7 anddetects the distance of a movable stage table 6 using variations inposition of a reflector 8 on the stage table.

A laser power supply 1 drives a gas laser light source 2 to generatelaser light 3, which is in turn reflected on a beam bender 4 and thenintroduced into an interferometer 5. The optical path is divided intotwo paths inside the interferometer 5. One of the optical paths is themeasurement path in which the light reaches the reflector 8 on the stagetable 6 and is then reflected thereon to return to the interferometer 5.The other optical path is the reference path in which the light isreflected inside the interferometer 5. This example employs a four-foldoptical path (in which light travels twice between the interferometer 5and the reflector 8) for laser displacement measurement. The light beamsof the measurement path and the reference path are mixed in theinterferometer 5. Mixing the two light beams causes interference tooccur. The light having caused the interference is launched from theinterferometer 5 and then detected by a light detector 9. The lightdetector 9 detects the light and converts the detected amount of lightinto an electrical signal. A measuring board 10 converts the resultingelectrical signal into a coordinate value and then outputs the resultingvalue as a measurement value 13. Means for increasing displacementoutput value accuracy 11 corrects the measurement value 13 to output ameasurement value 23 with increased accuracy. A personal computer 12 forcollecting data and performing control captures the value to performfeedback to the position of the stage and a correction mechanism.Incidentally, the measuring board 10 is generally called a counter boardor an axis board, but is herein consistently referred to as themeasuring board.

The present invention relates particularly to a correction method andmeans, employed in the means 11 for increasing displacement output valueaccuracy. In this embodiment, this is hereinafter described asindependent processing means as shown in FIG. 2. This is because thisfunction is implemented in the form of electrical signal processing andcan be implemented by either hardware or software. Thus, it is madepossible to implement this function by incorporating the means 11 ashardware into the measuring board 10 or by incorporating the means 11 assoftware into the personal computer 12. It is also possible to useindependent hardware to implement an independent configuration as shownin FIG. 2.

The gas laser light source 2 employed in this embodiment is a He—Ne gaslaser for emitting laser light at a wavelength of 633 nm. Ahigh-frequency electromagnetic wave is applied to the gas forexcitation. Incidentally, a vacuum chamber can be employed to seal theinterferometer 5 and the subsequent portions (the interferometer 5, thestage table 6, and the reflector 8) therein to provide the measurementpath in a vacuum, thereby making it possible to prevent an error inmeasurement caused by variations in refractivity due to a fluctuation ofair or a change in humidity of air. Furthermore, to maintain theaccuracy of measurement, a multi-layered coating is applied to thecomponents such as the beam bender 4, the interferometer 5, and atransparent window which are attached to the wall of the vacuum chamberin order to prevent unnecessary multiple reflections.

The laser interferometer displacement measuring system configured assuch makes it possible to provide measurement values or coordinateoutputs at a resolution of 0.3 nm and a high sampling rate of 10 MHz. Aspecific signal example is shown in FIG. 3.

FIG. 3A shows variations in measurement value, measured using the systemconfiguration of FIG. 2 at a resolution of 0.3 nm and a 10 MHz samplingrate while the single-axis stage is moving at a constant speed (5 mm persecond). The movement at the constant speed of the single-axis stagecauses the measurement values to apparently move linearly with respectto time. However, it would be found that the measurement values, whenmagnified, are actually not on a straight line. FIG. 3B shows a signalobtained by subtracting a linear component and a slight accelerationcomponent (a parabolic component) from the signal of FIG. 3A with thescale of the vertical axis being enlarged. Incidentally, the originalsignal is the same as that of FIG. 3A. The horizontal axis has beenchanged from the time to the position (length) scale, however, the datais taken from the same time-interval. As can be seen from FIG. 3B, themeasurement value fluctuates in the range of about ±2 nm and variescyclically. The fluctuation has four cycles for about 630 nm.

FIG. 4 shows similar data that has been obtained at a differenttraveling speed of the stage. FIG. 4A shows time-dependent variations inmeasurement value provided when the single-axis stage moves at a speedof 40 mm per second. FIG. 4B shows a signal obtained by subtracting alinear component and a parabolic component from the signal of FIG. 4A.Other points are the same as those FIGS. 3A and B. As in FIG. 3, thestage looks as if it moves linearly in FIG. 4A. However, in FIG. 4Bwhere the linear and parabolic components have been eliminated from themeasurement values, it can be found that the measurement valuesfluctuate with an amplitude of about ±2 nm in a cyclic manner. Thefluctuation has four cycles in the range of about 630 nm, which aregenerally the same as those of FIG. 3. The wavelength of the He—Ne laseremployed as the laser light source has a wavelength of 633 nm and theaforementioned cycles are exactly consistent with that of thealternating light and dark pattern caused by the interference of thelaser light. This shows that the cycle is an error in measurement causedby variations in quantity of laser light at a high frequency in thelight detector 9.

Due to this error, the measurement of a displacement of 70 nm or greaterwould provide an absolute accuracy in the range of an error of about ±2nm. Thus, even with such a laser interferometer displacement measuringsystem with a nominal value of resolution of 0.3 nm, the value ofabsolute accuracy would be measured being deteriorated several times insome cases as described above.

To correct this deterioration, the waveforms obtained as shown in FIGS.3B and 4B may be pre-stored as a correction value to subtract thecorrection value from subsequently obtained measurement values. Thismakes it possible to increase accuracy. The cycle of fluctuationcorresponds exactly to that of the laser wavelength. Accordingly, basedon the value obtained by measurement, a cyclic value may be generatedcorresponding to the wavelength cycle of laser light and used as acorrection value. The correction may be either carried out giving a highpriority to a real-time property (the property of real-time processing)by means of hardware or may be implemented by software means inconjunction with a mechanism for calculating correction values (shown inEmbodiments 2 to 5).

A method for processing a signal to implement the correction is shown inFIG. 5. FIG. 5 shows an example of the means 11 for increasingdisplacement output value accuracy, by which the measurement value 13outputted from the measuring board 10 is captured to output ameasurement value 23 with increased accuracy. In accordance with thecaptured measurement value 13, a correcting value 20 is read from amemory device 19 and corrected by a adder/subtractor 22, then beingoutputted as the measurement value 23 with increased accuracy.Designated as 25 is a parabolic component extracting filter unit foroutputting a phase shifting value 17.

The correcting value 20 is generated as described below. First, a phaseadder 16 adds the phase shifting value 17 to the measurement value 13and then outputs the resulting value as a table reference address 18. Inaccordance with the table reference address 18, the memory device 19outputs the correcting value 20 stored therein. The memory device 19 hasdata stored therein, which is to be outputted as the correcting value 20and which provides a cyclic value corresponding to the wavelength cycleof laser light. The value can be set to a given one presetting value 21.It is desirable that the table reference address 18 be cyclic inaccordance with the cycle of the laser wavelength. In this regard, onlythe upper bits equal to or greater than the wavelength cycle can beignored when the measurement value of laser interferometry employs thoseoutputted digitally with two to the power of N being adopted as thewavelength cycle. More specific values to be stored in the table are thewaveforms (or one cycle of the waveform) shown in FIGS. 3B and 4B, whichmay be stored as they are. Besides this, a sinusoidal (sin) waveformhaving the same cycle can be employed to allow the correction to providethe same effect of increasing accuracy.

FIG. 1 shows the effect of increasing accuracy according to the presentinvention. FIG. 1 shows a case where the waveform of fluctuation of FIG.3B is subtracted by the oscillation component of a single sinusoidalwave as a correction value, thereby providing increased accuracy. Inthis case where the original waveform has been sufficiently tracked, itis found that accuracy can be increased about three times with respectto the original accuracy to be in the range of about ±0.6 nm. It ispossible to further improve this accuracy by optimizing the waveform ofthe correction value to be employed for the subtraction. This methodwill be described in Embodiments 3 to 5. In the Embodiments 2 and 3,shown is the configuration of a system for fitting a sinusoidal wave byautomatic tracking, the system being provided with an increased accuracyshown in FIG. 1.

Incidentally, in the configuration of this system, the memory device 19is not inevitably necessary. Values corresponding to the table may becalculated on the spot. In many cases, however, simultaneous use of amemory device may generally facilitate configuration of the system. Thisprovides such flexibility that processing can be performed at highspeeds with no operation time being required and any given values can beset without employing any operational equations. The phase adder 16 andthe phase shifting value 17 are also not essential but provide increasedflexibility and an advantage of not having to rewrite all values of thetable in the memory when it is desired to shift only the phase of thecorrection value.

Incidentally, take the operation of subtracting the linear andacceleration components from FIG. 3A to yield FIG. 3B, and the operationof converting FIG. 4A into FIG. 4B. These operations can be implementedspecifically by fitting the quadratic function expressed in the form ofequation 1 shown below to the measurement values by the least squaremethod in a statistical manner and then by subtracting the averagedisplacement value of the resulting fitting.

[Equation 1]

y(x)=ax ² +bx+c

where “x” is the horizontal axis and “y” is the vertical axis.

This fitting can be carried out by determining constants a, b, and cwith respect to “yi” and “xi” in accordance with equation 2 employing adeterminant in a statistical manner. That is, $\begin{matrix}{{\begin{pmatrix}a \\b \\c\end{pmatrix} = {\begin{pmatrix}{X4} & {X3} & {X2} \\{X3} & {X2} & {X1} \\{X2} & {X1} & N\end{pmatrix}^{- 1}\begin{pmatrix}{X2Y} \\{X\quad Y} \\{Y1}\end{pmatrix}}}{{{X2Y} = {\sum\limits_{i = 1}^{N}\quad ( {x_{i}^{2}y_{i}} )}},\quad {{X\quad Y} = {\sum\limits_{i = 1}^{N}\quad ( {x_{i}y_{i}} )}},\quad {{Y1} = {\sum\limits_{i = 1}^{N}\quad y_{i}}},{{X1} = {\sum\limits_{i = 1}^{N}\quad x_{i}}},\quad {{X2} = {\sum\limits_{i = 1}^{N}\quad x_{i}^{2}}},\quad {{X3} = {\sum\limits_{i = 1}^{N}\quad x_{i}^{3}}},{{X4} = {\sum\limits_{i = 1}^{N}\quad x_{i}^{4}}}}} & \lbrack {{Equation}\quad 2} \rbrack\end{matrix}$

where “yi” is the measurement value either in the range of about fourcycles before and after the desired center of correction or in the rangeof about six cycles before or after the point, and “xi” is thecoordinate of each point.

The determined a, b, and c are substituted for the quadratic equationagain to determine “y” with respect to the point “x”, the error of whichis to be determined. A parabolic component extracting filter 25 performsthis processing. The operation can be implemented by subtracting the “y”determined as such from “yi”.

Summarizing the aforementioned procedure, first, the measurement values13 that are outputted continuously from the measuring board 10 arefitted by a quadratic function to subtract a fitted average displacementfrom an actual measurement value, thereby determining the fluctuationerror shown in FIGS. 3B and 4B. With this fluctuation error beingemployed as a correction value, the correction value is subtracted fromsubsequent measurements, thereby making it possible to provide ameasurement value which has a fluctuation error cancelled out and isincreased in accuracy.

FIG. 6 is a view showing an exemplary circuit for specificallyimplementing this by using a hardware-wise circuit configuration. Themeasurement value 13 from the laser interferometer displacementmeasuring system is divided into upper bits 14 corresponding to the wavecycle number of the laser and lower bits 15 indicative of the positionof decomposing one wavelength cycle. Among these lower bits, only thesignal of the lower bits is extracted and added to the phase shiftingvalue 17 at the phase adder 16. Thereafter, the phase value 18 aftershifting is inputted as the address of a dual port RAM (Random AccessMemory) 24. Incidentally, the dual port RAM is employed as the memorydevice here. With the presetting values 21 being pre-stored in the dualport RAM 24, the correcting value 20 is read in accordance with thephase value 18 after shifting. This correction value is added in theadder/subtractor 22 to the original measurement value 13 (the positioncoordinate signal produced by combining the upper bits 14 correspondingto the wave cycle number with the lower bits 15 indicative of theposition of decomposing one wavelength cycle), thereby providing thedisplacement value 23 after distortion correction. The correction valuefor addition may be normally sufficiently about 8 bits, and the upperbit portion insufficient for the number of bits (about 32 bits) of themeasurement value is extended by 8 bits for addition.

As described above, the measurement value 13 of the laser interferometerdisplacement measuring system is typically configured to count andoutput the wavelength cycle number in order to simplify theconfiguration of the hardware. This makes it possible to divide themeasurement value 13 into the bus of the upper bits 14 corresponding tothe wave cycle number of the laser and the lower bits 15 indicative ofthe position of decomposing one wavelength cycle. In the aforementionedembodiment, a cyclic correction is implemented with a simpleconfiguration by making use of this feature.

The correction value to be stored in the dual port RAM 24 is the same asthe operation for subtracting the aforementioned parabolic componentfrom the measurement value 13. In this figure, this calculation processis to be carried out using the personal computer 12. The procedure andequations to be processed are the same as those described in theforegoing.

Incidentally, as described above, the configuration of the measurementvalue corrector means for storing correction values on the rewritablememory allows a given cyclic function to be set as a correction value,thereby providing a high flexibility. Thus, by improving the calculationmethod of the correction value in the correction value calculation meansto be combined with the measurement value correction means, theconfiguration provides an advantage of allowing accuracy to be improvedpossibly to a level close to the limit that can be provided by thiscorrection method.

[Embodiment 2]

Configuration of Automatic Phase Tracking Means for Increasing Accuracy

FIG. 7 shows an exemplary configuration of automatic amplitude trackingdistortion correction means which performs feedback control on the phaseof cyclic function of a correction value to correct the distortion errorof a wave cycle.

The measurement value 13 obtained by the measuring board is captured andthen subtracted by the value that has passed through the paraboliccomponent extracting filter 25, thereby providing a distortion errorsignal 26. This calculation method is the same as that described inEmbodiment 1 using the determinant. In addition, the phase shiftingvalue 17 is added to the measurement value 13 in the phase adder 16 togenerate the table reference address 18. This value is employed as theinput to the memory device 19 for a generating cyclic function value andto a cyclic orthogonal function table 36 having a phase orthogonal tothe cyclic function. Incidentally, the memory device 19 may be a ROM(Read Only Memory) having fixed values stored therein. In addition, thememory device 19 and the cyclic orthogonal function table 36 are notlimited to the memory device but more generally may be replaced with afunction calculation mechanism as far as the mechanism can generatecyclic values. However, the shape of the function to be generated has tobe suitable for correction.

A cyclic function value 30 generated as described above and a cyclicorthogonal function value 37 are multiplied by the distortion errorsignal 26 at multipliers 31, 38 to obtain a correction strength, therebyproviding a correlation ratio 32 and a correlation ratio 39 for cyclicorthogonal function. These values are averaged with respect to time in atime-averaging filter 33 and an accumulator 40, thereby making itpossible to provide an average correlation strength of each signalcomponent.

An example of appropriate pairs of cyclic functions to be generated atthe memory device 19 and the cyclic orthogonal function table 36 isshown in FIGS. 8 and 9. The solid line of FIG. 8 represents a sinusoidal(sin) wave and the dotted line represents a cosine (cos) wave. When afour-fold optical path (in which light travels twice between theinterferometer and the reflector) for laser displacement measurement isused, such a function that takes on a cyclic value at each λ/4 may beemployed (where λ is the wavelength of the laser).For example, thesinusoidal wave is selected as the value to be set to the memory device19, while the cosine wave is selected as the value to be set to thecyclic orthogonal function table 36. When the sinusoidal wave generatedby the memory device 19 lags in phase behind the distortion error signal26, these settings provide a positive correlation with the cosine wavecomponent orthogonal in phase to the sinusoidal wave, thereby providingthe correlation ratio 39 for cyclic orthogonal function with a longertime for taking on a positive value than for taking on a negative value.The correlation ratio 39 for cyclic orthogonal function is added to theaccumulator 40, gradually increasing the phase shifting value 17 to beoutputted from the accumulator. Accordingly, the table reference address18 increases to thereby restore the delayed phase. On the contrary, whenthe sinusoidal wave leads in phase the distortion error signal 26, areverse operation is carried out to cause a decrease in the address.

This feedback allows the phase of the sinusoidal wave outputted from thememory device 19 to be always consistent with the phase of thefundamental wave component of the distortion error signal 26. Under thiscondition, oscillatory component of the sinusoidal wave or a correctionvalue is outputted from a multiplier 35 in accordance with an averagedcorrelation intensity 34 provided by the time-averaging filter 33. Thecorrecting value 20 is subtracted from the original measurement value 13in the adder/subtractor 22 to provide the displacement value 23 afterdistortion correction.

Incidentally, as can be seen from FIGS. 3B and 4B, the waveform of thedistortion signal appearing in the distortion error signal 26 takes onthe shape of a triangular wave rather than a sinusoidal wave. By using apair of triangular waves (each shown by a solid and dotted line in FIG.9) having phases different from each other by 90° in place of thesinusoidal and cosine waves shown in FIG. 8, it is possible to carry outcorrection more efficiently. Thus, a flexibility is given to theselection of a cyclic function.

Incidentally, the automatic phase tracking control can also beimplemented with the configuration shown in FIG. 6 or a combination ofhardware and software means. The phase adder 16 prepared in front of thememory device 19 can be used to replace the aforementioned phasefeedback processing by software-wise processing on the personal computer12 to thereby implement the distortion correction processing shown inFIG. 7 with the configuration of FIG. 6. In this case, although nomechanism is available for adjusting amplitudes, this mechanism can beimplemented by choosing several cyclic function values, which havedifferent amplitudes and are prepared in the dual port RAM 24, inconjunction with the presetting values 21.

[Embodiment 3]

Configuration of Automatic Amplitude Tracking Distortion Corrector Means(1)

FIG. 10 shows an exemplary configuration of automatic amplitude trackingdistortion corrector means for correcting the distortion error of ameasurement value by automatically controlling the amplitude of twocyclic functions.

The measurement value 13 provided by the measuring board is captured andthen subtracted by the value that has passed through the paraboliccomponent extracting filter 25 to thereby provide the distortion errorsignal 26. This calculation method is the same as that described inEmbodiment 2. On the other hand, the measurement value 13 is employed asit is to be inputted to the memory device 19 having the two cyclicfunctions therein and to the cyclic orthogonal function table 36. Eachof the generated cyclic function value 30 and the cyclic orthogonalfunction value 37 is multiplied by the cyclic orthogonal function table36 at each of the multipliers 31, 38, thereby providing the correlationof each cyclic function component included in the error signal. Thesecorrelations represent the correlation ratio 32 and the correlationratio 39 for cyclic orthogonal function. Each of these values is allowedto pass though the time-averaging filter 33, thereby providing theaveraged correlation intensity 34 and an averaged correlation intensity41 for the cyclic orthogonal function, which represent the magnitude ofeach cyclic function component. Each cyclic function value is added toeach other in proportion to the magnitude of each component. Suppose acombination of a sinusoidal and a cosine wave is used as the cyclicfunction. In this case, a sinusoidal wave having any phase can beexpressed with a linear superposition of these two waveforms. Thus, thephase feedback described in Embodiment 2 with reference to FIG. 7 is notalways necessary and the same effect can be provided only by suchamplitude control.

The amplitude of the cyclic function value 30 and the cyclic orthogonalfunction value 37 is varied in accordance with the signal intensity ofeach component or the averaged correlation intensity 34 and the averagedcorrelation intensity 41 for cyclic orthogonal function, thereby makingthe resulting amplitude consistent with the magnitude of each cyclicsignal component included in the distortion error signal 26. This iscarried out with multipliers 35, 42. As described above, a cyclicfunction value 43 and an orthogonal cyclic function value 44 areprovided as the output of the multipliers. A sum correction value 46that is obtained by adding each signal component of the cyclic functionsin a full adder 45 is subtracted from the original measurement value 13at the adder/subtractor 22, thereby providing the displacement value 23after distortion correction with cyclic distortion being eliminated.

As described above, it is possible to configure automatic distortionerror corrector means for tracking phases without employing a feedbackportion for tracking phases. Incidentally, the two cyclic functions usedhere, or the sinusoidal and cosine waves, are the same as those shown inFIG. 8 (by a solid and dotted line).

[Embodiment 4]

Configuration of Automatic Amplitude Tracking Distortion Corrector Means(2)

FIG. 11 shows an exemplary configuration of automatic amplitude trackingdistortion corrector means which is configured by expanding theconfiguration described in Embodiment 3 to automatically control theamplitude of three or more cyclic functions in order to provide furtherincreased accuracy.

Here, the sinusoidal and cosine waves and a group of harmonics of thesecyclic functions shown in FIG. 12 are used in place of the twosinusoidal wave and cosine waves, shown in FIG. 8, used in Embodiment 3.

As shown in FIG. 11, the automatic amplitude tracking distortioncorrector means is provided with a number of mechanisms, arranged inparallel to each other corresponding to the number of cyclic functionsused, for generating the two cyclic functions shown in FIG. 10. Insteadof the memory device 19 and the cyclic orthogonal function table 36,this configuration employs the memory device 19 and a plurality ofcyclic orthogonal function tables 47, 48. Although FIG. 11 shows onlythree mechanisms, a required number of mechanisms corresponding to thenumber of cyclic functions used are arranged in parallel to each other.The configuration subsequent to the full adder 45 is the same as that ofFIG. 10.

The sinusoidal and cosine waves and harmonics of these cyclic functionsare mathematically orthogonal to each other. This provides an advantageof allowing the intensity of amplitude corresponding to each componentto be independently calculated only by the calculation of correlationusing an accumulator.

Using a group of orthogonal functions as described above would make itpossible to generate a given cyclic function only by the assignment ofweights to and addition of each cyclic function. The orthogonality makesit possible to separately determine the intensity of the componentcorresponding to each function by means of an accumulator. Furthermore,the value of three or more cyclic functions including harmonics thereofcan be added to thereby track a distortion error signal having finerirregularities and reproduce the shape of the distortion error signalwith a higher fidelity. This provides an advantage of reducing aresidual or a distortion error by subtraction to thereby increaseaccuracy.

[Embodiment 5]

Configuration of Combined Corrector Means of Automatic AmplitudeTracking and Phase Tracking

FIG. 13 shows an exemplary configuration of combined automaticdistortion corrector means, in which automatic amplitude trackingcontrol and automatic phase tracking control are combined with eachother using a plurality of orthogonal cyclic functions.

In this configuration, the phase tracking feedback portion shown in FIG.7 and the amplitude control of the plurality of cyclic functions shownin FIG. 11 are combined with each other. The flow of signal and functionof each portion are the same in each of the portions bearing the samereference numbers of the embodiments 2 and 4. With such a configuration,it is possible to select a given group of orthogonal cyclic functionsother than sinusoidal and cosine waves as the group of cyclic functionsto be used.

An example of a cyclic function to be used in this embodiment includes agroup of orthogonal harmonics of a triangular-wave cyclic function. Thetriangular wave having been shown in FIG. 9 (by a solid line) isexpressed in the form of the following equation 3. That is,

[Equation 3]

y=Tri(x)

where x is the horizontal axis and y is the vertical axis.

With the group of orthogonal harmonics of a triangular-wave cyclicfunction being expressed in the form of equation 4 (where n is a naturalnumber), each of the harmonic cyclic functions having n=1 to 8 isexpressed in the form of equation 5.

[Equation 4]

y=orthoTri(nx)

[Equation 5]

orthoTri(1x)=Tri(1x)

orthoTri(2x)=Tri(2x)

orthoTri(3x)=Tri(3x)+{fraction (1/9)}Tri(1x)

orthoTri(4x)=Tri(4x)

orthoTri(5x)=Tri(5x)−{fraction (1/25)}Tri(1x)

orthoTri(6x)=Tri(6x)+{fraction (1/9)}Tri(2x)

orthoTri(7x)=Tri(7x)+{fraction (1/49)}Tri(1x)

orthoTri(8x)=Tri(8x)

These functions are specifically shown in FIG. 14. All of thesefunctions look a triangular-wave harmonic cyclic function with thevertex of each triangular wave being slightly shifted vertically due totheir orthogonality. Each of these orthogonal functions is selected soas to provide zero to the integration of the product of one function andanother over one cycle in the interval from 0 to λ/4.

The cyclic orthogonal function table 36 may generate a triangular-wavefunction of n=1, shown by a dotted line in FIG. 14, or a cosine-wavefunction of n1, shown by a dotted line in FIG. 12.

The method shown in Embodiment 4 requires both harmonic cyclic functionsof sinusoidal and cosine waves to provide phase flexibility to increaseaccuracy, thereby requiring an arrangement for generating a number ofcyclic functions. In contrast to this, from the viewpoint of phase, themethod shown in this embodiment simplifies the configuration only by theaddition of one feedback system to reduce the number of cyclic functionsby one-half. Furthermore, the method does not need to be bound to afunction such as a sinusoidal wave that can provide a given shift inphase by superposition, thus making it possible to employ from thebeginning a triangular wave close to a distortion error wave as a cyclicfunction. This makes it possible to reproduce efficiently the waveformof a distortion error signal including a number of triangular wavecomponents by means of a relatively small number of cyclic functionvalue generating means (the memory device 19 or the cyclic orthogonalfunction tables 47, 48). Thus, this provides an advantage of simplifyingthe configuration and reducing costs.

Incidentally, in this automatic phase tracking distortion correction,the feedback control can be stabilized by setting feedback time to avalue slightly shorter than the time constant of the time-averagingfilter 33 for use in amplitude control. Here, the feedback time forfeedback adjustment to provide phase consistency is determined by theintegral time constant of the accumulator 40.

[Embodiment 6]

Configuration Combined with Noise Reduction Averaging Means

FIG. 15 shows an exemplary configuration having a combination ofdistortion corrector means 27 described in Embodiments 1 to 5 andaveraging unit (averaging means) 28 for processing noise, allowing formeasurement at higher resolution than the minimum resolution of theoriginal measurement value 13.

The averaging means 28 receives and then allows the displacement value23 after distortion correction, provided by the distortion correctormeans 27, to be averaged for output by the method of moving averages inthe digital averaging unit incorporated therein over the same averagetime as the excitation cycle of lasing. More specifically, the hardwareconfiguration we have used provides the displacement value 23 afterdistortion correction in the form of coordinates data of 32 bits in a0.1 μs cycle. The value is averaged by the method of moving averages bymeans of hardware over an average time of 14.6 μs, which is the same asthe excitation cycle of lasing.

FIG. 16 shows the frequency spectrum of a signal of the displacementvalue 23 after distortion correction with the stage at a standstill. Ascan be seen in the figure, in addition to fine noise around 1 MHz, ahigh sharp noise component corresponding to the excitation frequency ofthe gas laser appears frequently near 68 kHz. Accordingly, for example,a filter having such a frequency characteristic as shown in equation 6below is used for averaging. $\begin{matrix}{\frac{\sin ( {2{\pi\Delta}\quad {\tau \cdot f}} )}{2{\pi\Delta}\quad {\tau \cdot f}}} & \lbrack {{Equation}\quad 6} \rbrack\end{matrix}$

where Δτ is the time constant of the filter and f is the frequency.

Specifically, a moving average filter may be employed as the filterhaving the characteristic shown in the above equation. The average timecan be determined to be 14.6 μs corresponding to the cycle of 68 kHz,thereby making it possible to eliminate the excitation noise of theaforementioned gas laser. Incidentally, the aforementioned average timecorresponds to 2 Δτ.

The averaging filter produces a response delay and phase shift andtherefore the distortion correction processing described in Embodiments1 to 5 has to be performed before this averaging processing. For thisreason, to use the averaging process in combination with the distortioncorrection processing, the distortion correction processing has to beperformed before the averaging processing as shown in FIG. 16.

Furthermore, suppose the system is configured such that only themeasurement value 29 with increased accuracy or a final output can bedetected externally as coordinates output value. In this case, since thephase shift occurs as described above by allowing the value to passthrough the averaging means 28, the system is desirably configured toallow the value to bypass the averaging means 28 at the time ofcalculating a correction value. Similarly, at the time of calculating acorrection value, the reference of the output value that has beencorrected with the previous correction value would make the calculationof the correction value further complicated. To prevent this, it wouldprovide an increased flexibility that the distortion corrector means 27itself can control whether or not the correction value is to be added.

Incidentally, such a prediction of distortion error (calculation of acorrection value) would work properly only when the stage is movingsmoothly in a continuous manner at a given constant speed (morespecifically at 2 mm per second or greater). Use of a continuouscoordinates output value measured at some other time would cause theerror contained in the value to be increased due to the mechanicalvibration, acceleration, or deceleration of the stage. Suppose the stagemoves at a given constant speed or less in the automatic trackingcorrection processing of a distortion error shown in Embodiments 2 to 5.When the stage moves at a lower speed, it is desirable to provide meansfor the time-averaging filters 33, 40 shown in FIGS. 7, 10, 11, and 13either to lock the average value without referencing the input value orto reference on a priority basis the input value as the input of theaveraging processing only when the stage moves at a given speed orgreater. For this purpose, for example, as shown in FIG. 17corresponding to FIG. 7, the system may be configured to differentiatethe measurement value 13 with a derivation filter 50 and issue an enablesignal for activating the time-averaging filters 33, 40 only when thevalue (the traveling speed of the stage) exceeds a certain giventhreshold value.

In addition, from that viewpoint, in the system employing the means forincreasing displacement output value accuracy according to the presentinvention, it is desirable for the stage or a measurement subject to beable to move smoothly in a continuous manner at an apparently constantspeed (more specifically, at a speed variation ratio of 0.05% or less)at the time of setting or resetting the correction value. It is thusdesirable to incorporate the movement of the stage and the measurementtarget subject and the measurement processing for this purpose into theprocess (procedure) to be performed at the time of initialization andcorrection. That is, the process (procedure) can be implemented only inthe system having a laser interferometer displacement measuring systemintegrated with driving means. On the other hand, smooth movement of thestage would not necessarily require a constant speed thereof. A nearlyconstant speed would make it easier to carry out an error calculation bythe subtraction of a parabolic component with accuracy and allow for alinear fitting instead of a parabolic fitting, thus facilitatingcalculation.

As shown in the foregoing, the method according to the present inventionallows a laser interferometer displacement measuring system to beprovided with a reduced cyclic measurement error caused by theinterference effect of laser, thereby implementing a higher absolutemeasurement accuracy than before. Incidentally, in the foregoing, thepresent invention has been explained with reference to an example with afour-fold optical path (in which light travels twice between theinterferometer 5 and the reflector 8) for laser displacementmeasurement, employing λ/4 as the fundamental cycle of a cyclic function(where λ is the wavelength of the laser).

With a two-fold optical path (in which light travels once between theinterferometer 5 and the reflector 8) for laser displacementmeasurement, the fundamental cycle of a cyclic function is λ/2. Ingeneral, the fundamental cycle of a cyclic function is λ/n with ann-fold optical path (in which light travels n times between theinterferometer 5 and the reflector 8) for laser displacementmeasurement.

According to the present invention, it is possible to provide improvedmeasurement accuracy to devices employing a laser interferometerdisplacement measuring system. Thus, the present invention is applicableto mechanisms or devices, which require among other things aconsiderably high absolute accuracy, such as a single-axis stage, an X-Ystage, a multi-axis stage, an electron beam lithography apparatus, astepper for fabricating semiconductors, a fine patterning equipment,precision patterning equipment, metal machining equipment, ceramicmachining equipment, mask pattern transfer equipment, mask patterningequipment, an electron-beam scanning microscope with a displacementmeasurement function, a transmission electron microscope, andnon-contact shape measurement equipment.

The laser interferometer displacement measuring system has originally ahigh absolute accuracy in the range of a longer distance than thewavelength of laser light, and in conjunction with the presentinvention, the absolute accuracy can be improved in the scale of thefrequency or less. This makes it possible to provide high accuracydevices with improved machining accuracy.

Suppose the distortion correction processing according to the presentinvention is performed in accordance with a measurement result providedby the laser interferometer displacement measuring system upon controlof movement of the stage or a measurement subject. In this case,performing the distortion correction processing on feedback controlwould make it possible to prevent the occurrence of unnecessary lasingby a cyclic measurement distortion component in the laser displacementmeasurement, thus providing an advantage of facilitating readily thestability of control.

[Embodiment 7]

Configuration of Laser Interferometer Displacement Measuring SystemEmploying Phase Detection

FIGS. 18 to 22 show the exemplary configuration of a laserinterferometer displacement measuring system employing a phase meterequipped with a phase tracking circuit (PLL or Phase-Locked Loop). Theexemplary configurations of FIGS. 18 to 21 show a laser interferometerdisplacement measuring system in which the displacement of a subjectbody having the reflector 8 causes a four-fold change in length of theoptical path to the optical path length of the measurement path beambetween the interferometer and the subject body. FIG. 22 shows anoptical system in which a displacement of the subject body causes aneight-fold change in length of the optical path. Incidentally, theoptical system with either the four-fold or eight-fold optical path hasthe same configuration of the signal processing system subsequent to thelight detector 9.

Now, described below is the principle of the aforementioned laserinterferometer displacement measuring system according to the presentinvention. First, referring to FIG. 21, the configuration of the opticalsystem is described.

The laser light 3 emitted from the gas laser light source 2 impingesupon a polarizing beam splitter 51 a. The laser light is linearlypolarized and provided with a polarization at an angle of 45 withrespect to the polarizing beam splitter 51 a. The laser light is splitinto two beams: one beam (reference path beam) to be reflected on thepolarizing beam splitter 51 a towards a retroreflector 52 a and theother beam (measurement path beam) to be transmitted as it is towards apolarizing beam splitter 51 b. The reference path beam reflected towardsthe retroreflector 52 a is reflected twice in the retroreflector toreturn to the optical path on the upper side of the polarizing beamsplitter 51 a, then being reflected to the right at the reflecting planeof the polarizing beam splitter 51 a to impinge upon the light detector9. On the other hand, the measurement path beam transmitted towards thepolarizing beam splitter 51 b passes through the subsequent polarizingbeam splitter 51 b as it is and then a quarter wave plate 53, beingpolarized circularly to reach the reflector 8. Upon being reflected onthe reflector 8 and then passing again through the quarter wave plate53, the measurement path beam, circularly polarized, is rotated by 90°with respect to the original polarization. The laser light with apolarization rotated by 90° is reflected on the reflecting surface ofthe polarizing beam splitter 51 b towards a retroreflector 52 b. Then,this light is reflected twice in the retroreflector 52 b to return tothe optical path on the upper side of the polarizing beam splitter 51 b.The light, still having the polarization rotated by 90°, is reflected tothe left at the reflecting plane of the polarizing beam splitter 51 band then passes through the quarter wave plate 53 to reach again thereflector 8 being circularly polarized. Then, upon being reflected onthe reflector 8 to pass again through the quarter wave plate 53, thislight is rotated again by 90° to be provided with the originalpolarization and then passes through the polarizing beam splitter 51 bas it is. Then, the light passes through the polarizing beam splitter 51a as it is to reach the light detector 9. At this time, the light ismixed with the reference path beam that has first reached there via theretroreflector 52 a to optically interfere with each other. A light beamprovided with an alternating light and dark pattern by the interferenceof light is collected by and detected in the light detector 9.Incidentally, it is to be understood here that the light detector 9 alsohas the function of an optical detector. It is also to be understoodthat a detected signal of the optical detector or a signal indicative ofan analog quantity proportional to the amount of light received isreferred to as a signal indicative of the amount of light received.

The path between the quarter wave plate and the reflector 8 is referredto as a measurement path 70. Since the measurement path beam travelstwice along the measurement path 70, a displacement in the reflector 8causes a four-fold variation in the length of the optical path, throughwhich the measurement path beam passes, with respect to the displacementof the reflector 8. This causes a displacement of λ/4 in the reflector 8to produce a phase difference between the measurement and reference pathbeams to thereby allow interference of laser light to occur, resultingin a blink of the light detected by the light detector 9. The count ofthe blinks makes it possible to measure a variation in length of themeasurement optical path. For light of wavelength 633 nm, one blinkcorresponds to a displacement of 158 nm. To measure a displacement withan accuracy of 1 nm, it is necessary to measure this light blinkaccurately at a higher resolution than one wavelength. For this purpose,phase detector means such as a phase meter is generally provided usingsynchronization detection such as with a phase-locked loop (PLL). Bythis signal processing, the phase variation of the measurement andreference path beams is accurately measured.

Now, the configuration subsequent to the light detector 9 is explainedwith reference to FIG. 18.

Variations in intensity of the blinks detected by the light detector 9are inputted into phase detector means 55 as a photodetector detectionsignal 54. As described above, the phase detector means 55 measuresaccurately the phase difference between the measurement and referencepath beams in accordance with the variation in intensity of interferencelight corresponding to the light blinks in order to determine a phasevalue 56. The amount of variations in the phase value 56 is accumulatedin accumulator means 57 to determine an accumulated value 58corresponding to the displacement of the reflector. The accumulatedvalue 58, close to the measurement value 13, has an error caused by theoptical system or circuitry. To configure a high accuracy laserinterferometer displacement measuring system, it is necessary toeliminate errors contained in the accumulated value 58 by correction.The laser interferometer displacement measuring system according to thepresent invention is provided with a correction function for reducingthese errors. As shown in Embodiment 1, the aforementioned accumulatedvalue 58 often includes an error mainly composed of wavelength λ of thelaser light or harmonics thereof. In general, in a laser interferometerdisplacement measuring system, the aforementioned accumulated value 58is provided in the form of the sum of an integral multiple of wavelengthand the dividing ratio of one wavelength. This is because the laserinterferometer displacement measuring system counts a distance by thenumber of wavelengths. Therefore, it is generally easy to obtain cycliccomponents of wavelength λ of the laser light from the aforementionedaccumulated value. The cyclic components of wavelength λ of the laserlight are detected synchronously from the accumulated value 58. When avibration component is found which has a frequency that never appearsunder a normal condition, it can be determined mechanically that thecomponent is the error of the laser interferometer displacementmeasuring system. A correction value generator means 61 performs theforegoing. The approach described in Embodiments 1 to 6 can be used forthis purpose. A correction value 60 obtained by the correction valuegenerator means 61 is added to the aforementioned accumulated value 58by means of a correction value adder 59, thereby providing themeasurement value 13 with increased accuracy.

Incidentally, as an alternative configuration to that of FIG. 18, thereis available another configuration, as shown in FIG. 19, in which thecorrection value adder 59 is inserted in between the phase detectormeans 55 and an accumulator (accumulator means) 57 to generate thecorrection value 60 with the phase value 56 being employed as a variableand then the resulting value is added to the phase value 56 itself. Onthe other hand, FIG. 20 shows a configuration in which the correctionvalue adder 59 is arranged behind the accumulator means 57 to add theresulting value to the accumulated value 58. However, in theseconfigurations, the phase value 56 is employed as a variable to generatethe correction value 60, thereby limiting to less than λ/4 the cycle ofthe correction value to be added. The configuration shown in FIG. 18provides an advantage of being available for an error of a decreasedcycle.

Now, the cause of occurrence of measurement error will be explainedbelow referring back to FIG. 21.

The configuration of the optical system of the laser interferometerdisplacement measuring system shown in FIG. 21 usually employs lenscomponents that are applied with non-reflective coating to preventunnecessary reflection of light. However, an error or unevenness inthickness of the non-reflective coating would often result in 1 to 2%reflection of light at each of the boundaries between air and lenscomponents. Among the reflected beams of light, surface-reflected light71 reflected on the surface of the quarter wave plate 53 opposite to thereflector affects directly the measurement error of this optical system.Except that the surface-reflected light 71 does not travel along themeasurement path 70, the surface-reflected light 71 is rotationallypolarized at the surface on the reflector side of the quarter wave plate53 in the same manner as a usual measurement path beam and then travelsalong the same optical path to be detected by the light detector 9. Thesame thing occurs when light passes through the quarter wave plate forthe second time, causing surface-reflected light 72 to be produced.Suppose a variation in length of the optical path, caused by themovement of the subject body, of the measurement path beam of each lightcomponent is expressed to be N times the displacement of the subjectbody. Thus, the beams of surface-reflected light 71, 72, which travelonce (N=2) and are mixed with a measurement path beam (N=4) that issupposed to travel twice along the measurement path 70, are detected bythe light detector 9, causing an error in the displacement measurement.

The same thing happens when light travels three times or more along themeasurement path as shown in FIG. 22. FIG. 22 shows an example of anincreased number of travels of light along the measurement optical pathto increase the resolution of measurement, showing an exemplaryconfiguration of an optical system in which a measurement path beamtravels four times along the measurement path 70. In this case, 4 to 8%of light traveling three times (N=6), mixed with the light travelingfour times (N=8), is detected by the light detector 9, causing an errorin the displacement measurement. Similarly, light of N=4 or N=2 is alsocontained to cause an error of the measurement.

Contemporary techniques cannot completely eliminate these optical errorsin the optical system. Therefore, these errors are desirably eliminatedby means of signal processing. More specifically, the configuration asshown in FIG. 18 can be used for this purpose. As described above, lightof N=8 is mixed such as with light of N=6 and N=4. For example, mixingof light N=2 and N=4 would cause a beat to occur due to a component waveof N=(4+2)/2=3 and a component wave of N=(4−2)/2=1. This causes an errorof a long cycle corresponding to N=1(i.e., a cycle of λ). To eliminatethis error, it is desirable to generate the correction value 60 not withthe phase value 56 detected in the cycle of N=8 but with the accumulatedvalue 58 being employed as a variable. In addition, an error in theshape of a triangular wave as shown in FIG. 4 means that a number of2N-harmonic sinusoidal components are contained in addition toN-harmonic components. In order to eliminate these components, it isdesirable that a plurality of sinusoidal waves to be employed as acorrection value should contain 1 to 2 n harmonic sinusoidal wavecomponents. In this regard, the correction method shown in Embodiment 4is available which employs the sum of a plurality of harmonic cyclicfunctions as a correction value. Actual examples of the effects of thiscorrection method are shown in FIG. 23. The measurement accuracy in therange of ±0.6 nm as shown in FIG. 1 is further increased up to the rangeof ±0.4 nm by employing the sum of 1 to 2 harmonic sinusoidal and cosinewaves as the correction value 60. Configuration for correcting each ofthe 1 to 2 n harmonic components makes it possible to further increaseaccuracy as mentioned above when compared with the correction of asingle sinusoidal component.

On the other hand, the configuration of the automatic tracking correctormeans as shown in Embodiments 2 to 5 will inevitably produce a timedelay from the time of detecting these cyclic error components resultedfrom the movement of the stage until the components converges on anoptimum correction value. More specifically, the system is started, thestage is then move, a signal of a λ cycle component is detected, acorrection value is calculated or adjusted, and thereafter outputted isa measurement value with increased accuracy provided by the correctionvalue. Accordingly, immediately after the system has been started, themovement of the stage will first allow measurement values containing anumber of error components of a λ cycle to be outputted, and after awhile, corrected measurement values with less errors will be outputted.The delay time is determined by the feedback time of the automatictracking system as described in Embodiment 5. However, when the stage isat a standstill or moves slowly, it is impossible to distinguish anoscillation signal resulted from interference of light from a vibrationsignal caused by mechanical vibration. In this regard, required is meansfor enabling or disabling the update of correction values based on thetraveling speed of the stage. This is more specifically shown in FIG.26. Derivator means 63 is allowed to detect the speed of the stage basedon a variation in the accumulated value 58, and then an update enablingsignal 64 is outputted to update the correction value when the speed hasexceeded a given speed. FIG. 27 shows an exemplary configuration inwhich a λ-synchronous signal extractor (λ-synchronous signal extractormeans) 62 detects an error of a wavelength λ cycle that is included inthe measurement value 13 and remains after correction, and feedbackcontrol is performed to minimize the error. The update enable signal 64controls the update of the correction value in accordance with the speedof the subject body (stage). This makes it possible to preventaccidental update of the correction value at the time of low speeds.Such a configuration allows the error of a λ cycle to be continuouslysuppressed even after the stage has been slowed down, thereby making itpossible to improve the measurement accuracy upon measurement with thestage at a standstill.

As described above, the configuration of the laser interferometerdisplacement measuring system according to the present invention ischaracterized by comprising calculation means for calculating acorrection value for correction of the measurement value of displacementof a subject body and an addition mechanism for adding the correctionvalue. The configuration is also characterized by adding or subtractinga cyclic correction value that is in phase with the fundamentalwavelength λ of laser light. A laser interferometer displacementmeasuring system with a multi-fold optical path can also have ameasurement value with increased accuracy. This configuration comprisesinevitably the calculation means for calculating and adder/subtractormeans for adding or subtracting a correction value, which follow phasedetection. An error signal included in the measurement value isdetermined and the resulting error signal is added or subtracted as acorrection value, thereby providing the measurement value with increasedaccuracy. The feature of the error signal makes it possible to implementhigh accuracy non-contact displacement measurement in the range oferror±0.4 nm or less by subtracting 1 to 2 n harmonic wave components ofthe wavelength λ of laser light, independent of n, using a cycliccorrection value that employs the wavelength λ of laser light as afundamental cycle. Correction value generator means can employ a methodfor generating a cyclic correction value having the cycle of thewavelength λ of laser light, with a phase value or an accumulated valuebeing employed as a variable. Furthermore, available for generatingcorrection values is an approach for synchronously detecting a frequencysignal component of the cycle of wavelength λ, which is contained in atime-dependent accumulated value. In addition, it is also possible toautomatically generate a correction value by automatic tracking so as tominimize the signal component with the cycle of wavelength λ, which iscontained in the corrected signal. In this case, since tracking iscarried out so as to reduce the signal component having a frequencycomponent of cycle λ contained in a measurement value after the subjectbody has moved, the frequency component of the measurement value varieswith time.

The frequency f of the signal produced at a cycle of wavelength λ isdetermined by the traveling speed v of the stage as f=Nv/λ (where N is anatural number) and can be therefore distinguished clearly fromvibration signals caused by other factors. With the averaging filter, acut-off frequency that is so set as to eliminate the component offrequency f would cause signals in the frequency band greater than f tobe attenuated at the same time, thereby eliminating signals which arecaused by other oscillation factors and are originally to be detected.In contrast, the corrector means according to the present invention ischaracterized in that only an error signal caused by interference oflight can be detected by synchronous detection such as lock-in detectionand eliminated, thereby making it possible to correct the error signalwithout attenuating signals which are caused by a mechanical vibrationand are originally to be detected. An actual example is shown in FIG.28. The displacement of the stage is measured during the movement of thestage and the resulting signals before and after correction areexpressed in the graphs of frequency spectrum. As shown in FIG. 28A,peaks of f=NV/λ are observed at equal intervals in the signal beforecorrection. On the other hand, in FIG. 28B, those peaks at equalintervals have been eliminated in the corrected spectrum but the signalsof other high frequency band components have not been attenuated. Thus,only such peaks of frequency f=Nv/λ in phase with wavelength λ can beselectively eliminated, thereby making it possible to carry outcorrection without attenuating the peaks of a frequency component causedby other factors. In general, when averaging applied to noise processingis carried out to attenuate a peak of a certain frequency, signal peaksin the band greater than the frequency are attenuated at the same time.In contrast, the measurement value provided by the laser interferometerdisplacement measuring system according to the present invention makesit possible to selectively eliminate optical noise of cycle λ, whileallowing the magnitude of vibration signals caused by mechanical factorsto remain unchanged. Use of the correction method according to thepresent invention causes an error of cycle λ in the signal indicative ofthe amount of light received that is detected by the light detector.Thus, the measurement of the frequency spectrum would provide an effectof correction as in the comparison of FIGS. 28A with 28B. The featurelies in that only those peaks corresponding to the frequency of cycle λare selectively attenuated in the spectrum with respect to backgroundcomponents. This is observed in such a manner that the peaks areattenuated relative to the baseline (background components) of aspectrum near the peak of a frequency f. The intensity ratio of the peakto the baseline is to be referred to as a relative peak intensity. Thatis, in increasing accuracy in the present invention, the relative peakintensity of a peak of f=Nv/λ is selectively suppressed or attenuated.In contrast to this, in noise processing by averaging (time averaging),a variation in gain of frequency is gradual, and thus the peak and thebaseline are attenuated by averaging generally with the same ratio atthe same time, thereby maintaining the relative peak intensity to aconstant magnitude. This can be clearly distinguished from theelimination of a signal of cycle λ according to the present invention.In addition, suppose the elimination of a signal of cycle λ according tothe present invention is combined with the averaging. Even in this case,since the relative peak intensity of the peak provided by averaging tothe baseline remains unchanged, a variation in the relative peakintensity occurs only in the elimination of a signal of cycle λ. Thismakes it possible to determine the availability of this approach inaccordance with relative intensity. In addition, suppose the frequencyspectrum of a measurement value is compared with a signal indicative ofthe amount of light received by the light detector. The peak of f=Nv/λof the signal indicative of the amount of light received or a frequencycomponent of N=n is a very signal provided by interference of light inaccordance with the principle of laser interferometer displacementmeasurement and corresponds to the component of a uniform linear motionof the stage, being outputted as a linear increase in measurement value.Therefore, frequency components of N≠n correspond to measurement errors.In other words, the corrector means according to the present inventionis characterized, when used, in that the peaks of N=1 to 2 n and N≠n areattenuated relative to the baseline.

However, since an error of cycle λ cannot be measured when the stage isat a standstill, the error of cycle λ is detected and corrected afterthe stage has been put into motion. This allows peaks to appear in thefrequency spectrum of observed measurement values during the firstmovement or acceleration of the stage. When a correction value has beenset or tracking has been performed to provide an appropriate value,these peaks are observed to have reduced.

The laser interferometer displacement measuring system according to thepresent invention is available for various types of systems andequipment describe in Embodiment 6, which are required for high accuracymachining, and is particularly useful for significantly increasing themeasurement or machining accuracy of systems and equipment that employ alaser interferometer displacement measuring system with a multi-foldoptical path.

[Embodiment 8]

An Exemplary Configuration of Reducing Projection Exposure Apparatus

FIG. 24 shows an exemplary configuration of reducing projection exposureapparatus (a stepper for semiconductors) that employs the laserinterferometer displacement measuring system according to the presentinvention. A stage for placing a wafer 81 and a photomask 82 thereon isprovided on an air spring vibration isolator 80 and protected fromexterior vibration. Exposure light 84 emitted from an exposure lightsource 83 passes through a shutter 85 and is then reflected on the beambender 4 to be introduced into the upper portion of the equipment.Subsequently, the exposure light 84 is expanded in diameter of the beamby means of a beam expander means 86 and passes through a lens 87 toilluminate the photomask 82. The photomask 82 is mounted on a photomaskstage 88 and adapted to be movable. The photomask stage 88 comprises thereflector 8. The position of the photomask stage 88 is measured by threelaser displacement measurement units 89 each comprising the laserinterferometer displacement measuring system, described in Embodiments 1to 7, which can detect the displacement and rotational error of thestage. Incidentally, an interferometer and a light detector, which arehoused inside the laser displacement measurement unit 89, can measurethe distance to a reflector. The exposure light that has passed throughthe photomask 82 is condensed to a given scale by means of a reducingprojection lens 90 and thereafter illuminated onto the wafer 81. Thiscauses the pattern on the photomask 82 is projected onto the wafer 81.The exposure time is controlled by opening or closing the shutter 85. Awafer stage 91 for placing the wafer 81 thereon comprises the reflector8. The positional and rotational errors of the wafer stage 91 aremeasured by means of the three laser displacement measurement units 89.It is preferable that the photomask stage 88 and the wafer stage 91 canbe moved by means of a motor and comprises an inching mechanism forinching such as a piezoelectric device. The accurate positionalrelationship between the wafer and photomask is detected with analignment detection unit (alignment detection means) 92. The alignmentdetection means 92 comprises light-emitting and light-receiving devicesand detects alignment patterns prepared on the wafer and the photomask,thereby making it possible to accurately measure a positional shift inpattern in the range of accuracy of about 2 nm or less. With employingthe first alignment signal as reference, the stage is repeatedly moved.The use of the laser interferometer displacement measuring system thatprovides a high measurement accuracy will implement an accuratealignment, thereby increasing the superposition accuracy of the patternto be exposed onto the wafer. Incidentally, a configuration adapted toemploy an evacuator means 93 for reducing the pressure of the entireequipment will make it possible to use ultraviolet light of a shortwavelength as an exposure light in a vacuum, thereby implementing finepatterning. It is also possible to prevent the effect of a variation inrefractivity of air in the measurement optical path, thereby making itpossible to further improve the superposition accuracy of a pattern.

In addition, an integrally configured exposure apparatus would make itpossible to control the equipment integrally including the movement ofthe stage. Thus, the stage can be moved in advance at the time ofinitializing the equipment to detect the error that occurs in the cycleof the wavelength of the measurement laser light and set a correctionvalue. In this case, it is not always necessary to update the correctionvalue in real time by automatic tracking or automatic feedback. Thisprovides an advantage of much more simplifying the configuration of themeans for generating and updating the correction value.

Among other things, the use of the laser interferometer displacementmeasuring system according to the present invention will provides aconsiderable advantage in manufacturing a fine circuit pattern, having a0.07 μm rule width or less, which requires a measurement accuracy in therange of below 2 nm. The wavelength of the exposure light required forthe exposure of this rule width is predicted to be 160 nm or less. Thelaser interferometer displacement measuring system is particularlyuseful for an exposure apparatus that employs such a short wavelength.

[Embodiment 9]

Exemplary Configuration of an Electron-beam Mask Drawing Apparatus

FIG. 25 shows an exemplary configuration of equipment, incorporating thelaser interferometer displacement measuring system according to thepresent invention, for drawing a lithography mask (reticle) with anelectron beam. The equipment comprises the air spring vibration isolator80 and is divided largely into three portions of an electron-beam gununit 100, a stage unit 120, and a control unit 130. A high voltage isapplied between an electron-beam gun 101 and an acceleration voltageelectrode 102, thereby generating an electron beam. A beam-shapingdeflector 103 directs this electron beam to an arbitrary point on afirst beam-shaping mask 104 to shape a beam for the first time. Theelectron beam that has passed through the first beam-shaping mask 104passes through a condenser lens 105 and a blanking voltage electrode106, thereafter being shaped again by a second beam-shaping mask 107.After having passed through the second beam-shaping mask 107, theelectron beam is deflected by a positioning deflector 108 and thencondensed towards a drawing point through an objective lens 109. Afterhaving passed through a secondary deflection electrode 110, the electronbeam is illuminated onto a master photomask 122 placed on an X-Y stage121. The illuminated point on the master photomask 122 is selected bythe aforementioned positioning deflector 108 and the aforementionedsecondary deflection electrode 110, and the voltage to be applied to theaforementioned blanking voltage electrode 106 is controlled to turn onor off the illumination of the electron beam, thereby drawing a desiredpattern on the master photomask. The control unit 130 controls thesesignals. The X-Y stage 121 can be moved to select the region on themaster photomask where the pattern is drawn. The position of the X-Ystage 121 is measured accurately with the laser displacement measurementunit 89. The laser displacement measurement unit houses theinterferometer and the light detector (photo-detector) for measuring thedisplacement of the reflector 8 on the X-Y stage 121. A motor is used todrive the X-Y stage. The aforementioned laser displacement measurementunit measures a control error with respect to the target position of theX-Y stage and the voltage applied to the secondary deflection electrode110 is controlled for deflection correction. A wafer height sensor 111is used for detecting a variation in height of the master photomask,adjusting the aforementioned objective lens, and controllingautomatically the focusing of an electron beam illuminated onto thewafer, thereby making it possible to draw a fine pattern with highresolution. A detection signal, measured by the laser displacementmeasurement unit 89, of the light detector is processed using the signalprocessing method of the laser interferometer displacement measuringsystem described in Embodiments 1 to 7, thereby making it possible toprovide a high accuracy measurement value for accurate positiondetection. This makes it possible to draw a pattern on the masterphotomask with an increased accuracy.

The laser interferometer displacement measuring system according to thepresent invention has an outstanding effect, among other things, on thecontinuous movement drawing in which an electron beam is illuminateddown to the wafer to draw a pattern with the X-Y stage beingcontinuously moved. When found is a difference between the targetposition of the stage and the current position measured by the laserinterferometer displacement measuring system, the aforementionedelectron beam lithography apparatus deflects the electron beam tocorrect the difference. Without the approach to increasing the accuracyof measurement values in the laser interferometer displacement measuringsystem according to the present invention, an error signal having acycle of wavelength λ of laser light is added to deflection correction.This measurement error directly turns to be an error in drawing apattern. In contrast to this, with the approach to increasing theaccuracy of measurement values in the laser interferometer displacementmeasuring system according to the present invention, the measurementerror of cycle λ is corrected and thereby a proper pattern is drawn. Inaddition, unlike the noise processing by averaging, since it is possibleto selectively eliminate only an optical error of the laserinterferometer displacement measuring system, other signals (caused suchas by mechanical vibrations) can be detected without impairing thefrequency band. This makes it possible to minimize a control errorcaused by a delay in response time. This allows for implementing a highdrawing accuracy.

In addition, an integrally configured electron beam lithographyapparatus would make it possible to control the equipment integrallyincluding the movement of the stage. Thus, the stage can be moved inadvance at the time of initializing the equipment to detect the errorthat occurs in the cycle of the wavelength of the measurement laserlight and set a correction value. In this case, it is not alwaysnecessary to update the correction value in real time by automatictracking or automatic feedback. This provides an advantage of much moresimplifying the configuration of the means for generating and updatingthe correction value.

As described above, the present invention makes it possible to correct ameasurement error corresponding to the wavelength cycle of laser light.Thus, the present invention can provide increased accuracy in the rangeof below 1 nm upon measurement of a displacement of a sample or a targetwork when employed for a high accuracy displacement measurement,instrumentation, and evaluation technique, which requires high absoluteaccuracy of the order of nanometer, or for precision and fine patterningtechniques such as semiconductor and master mask patterning.

What is claimed is:
 1. A laser interferometer displacement measuringsystem comprising a displacement measurement mechanism making use oflaser interference to produce a measurement value; and corrector meansfor adding a correction value to or subtracting the correction valuefrom said measurement value produced by said displacement measurementmechanism, wherein said correction value used by said corrector means isa cyclic correction value generated based on said measurement value andhaving a cycle corresponding to a wave cycle of laser light.
 2. A laserinterferometer displacement measuring system comprising: a displacementmeasuring mechanism making use of laser interference to produce ameasurement value; and corrector means for adding a correction value toor subtracting the correction value from said measurement value producedby said displacement measurement mechanism, wherein said corrector meanshas storage means for storing as said correction value a cycliccorrection value generated based on said measurement value and having acycle corresponding to a wave cycle of laser light, and wherein saidcorrection value is read out of said storage means in accordance withsaid measurement value and is added to or subtracted from saidmeasurement value.
 3. A laser interferometer displacement measuringsystem, comprising: a laser light source; an interferometer for dividinglaser light of wavelength λ emitted from said laser light source into areference path beam and a measurement path beam to interfere saidreference path beam with said measurement path beam having beenreflected from a subject body; a light detector for detecting the lightsubjected to the interference in said interferometer to produce ameasurement value; measurement value output means for converting adetection signal of said light detector into said measurement value andoutputting said measurement value, wherein a displacement of the subjectbody causes an n-fold variation in length of an optical path betweensaid interferometer and the subject body; and corrector means for addinga correction value to or subtracting the correction value from saidmeasurement value output by said measurement value output means, whereinsaid measurement value is a variable, and said correction valuegenerated based on said measurement value and used by said correctormeans is a cyclic function having a cycle of λ/n or a sum of a pluralityof cyclic functions having said cycle of λ/n as a fundamental cycle. 4.The laser interferometer displacement measuring system according toclaim 3, wherein the plurality of cyclic functions having said cycle ofλ/n as a fundamental cycle are the cyclic function having a cycle of λ/nand harmonic cyclic functions thereof.
 5. The laser interferometerdisplacement measuring system according to any one of claims 1 to 4,further comprising means for performing feedback control so as to carryout tracking adjustment of a phase and amplitude of the correctionvalue.
 6. The laser interferometer displacement measuring systemaccording to any one of claims 1 to 5, wherein averaging unit meanscapable of averaging over time is provided after said corrector means.7. A laser interferometer displacement measuring system comprising: adisplacement measurement mechanism making use of laser interference toproduce a measurement value; error signal component generating means foreliminating a constant speed component and an acceleration componentfrom said measurement value produced by said displacement measurementmechanism and generating an error signal component based on saidmeasurement value; storage means for storing said error signal componentfrom said error signal component generating means corresponding to saidmeasurement value; and means for allowing said error signal componentstored in said storage means to be added to or subtracted from saidmeasurement value of said displacement measurement mechanism as acorrection value.
 8. A laser interferometer displacement measuringsystem, comprising: a laser light source; an interferometer for dividinglaser light of wavelength λ emitted from said laser light source into areference path beam and a measurement path beam to interfere saidreference path beam with said measurement path beam having beenreflected from a subject body; a light detector for detecting the lightsubjected to the interference in said interferometer to produce ameasurement value; measurement value output means for converting adetection signal of said light detector into said measurement value andoutputting said measurement value, wherein a displacement of the subjectbody causes an n-fold variation in length of an optical path betweensaid interferometer and the subject body; and corrector means for addinga correction value generated based on said measurement value to orsubtracting said correction value from said measurement value output bysaid measurement value output means, wherein said corrector meanscomprises: means for storing or calculating, with said measurement valuebeing employed as a variable, said correction value as a cyclic functionhaving a cycle of λ/n or said cyclic function having a cycle of λ/n andharmonic cyclic functions thereof, error signal component generatingmeans for eliminating a constant speed component and an accelerationcomponent from said measurement value of said displacement measurementmechanism and generating an error signal component, adjustment means foradjusting an amplitude and a phase of said cyclic function so that saidcyclic function having a cycle of λ/n or a sum function of said cyclicfunction having a cycle of λ/n and harmonic cyclic functions thereoffits to said error signal component, and means for allowing a functionvalue of said cyclic function having a cycle of λ/n or a function valueof said sum function of said cyclic function having a cycle of λ/n andharmonic cyclic functions thereof to be added to or subtracted from saidmeasurement value.
 9. An apparatus comprising a stage for placingthereon and moving a sample or a subject work, drive means for drivingsaid stage, and a laser interferometer displacement measuring system formeasuring a position of said stage, wherein as said laser interferometerdisplacement measuring system, the laser interferometer displacementmeasuring system according to any one of claims 1-5, 8 and 8 isemployed.
 10. An apparatus comprising a stage for placing thereon andmoving a sample or a subject work, drive means for driving said stage,and a laser interferometer displacement measuring system for measuring aposition of said stage, wherein as said laser interferometerdisplacement measuring system, the laser interferometer displacementmeasuring system according to claim 7 is employed.